A class of lasers commonly used in optical communications systems is single longitudinal mode (SLM) lasers. These lasers tend to be semiconductor diode lasers and have a dominant transmission mode such that the vast majority of the radiation is transmitted at the wavelength of the dominant transmission mode. Typically, the dominant transmission mode emits at a power that is 30-40 dB greater than the next most powerful mode. The most common form of SLM laser is the Distributed Feedback (DFB) laser, in which a refractive-index grating is formed within the semiconductor device to enhance the Bragg reflection of a particular optical wavelength within the laser cavity. This causes the wavelength reflected by the grating to experience significantly greater gain than other potential longitudinal modes, causing the wavelength reflected by the grating to define the dominant transmission mode of the laser.
Other types of SLM laser are the Distributed Bragg Reflector (DBR) laser and the Vertical Cavity Surface Emitting Laser (VCSEL). DBR lasers incorporate an external Bragg grating that reflects radiation of a particular wavelength back into the laser cavity, this reflected wavelength defining the dominant transmission mode of the laser. VCSELs also use a Bragg grating to reflect light back into the cavity, but due to the VCSEL structure the grating is formed outside the gain region but is integrated within the same epitaxially grown structure.
SLM lasers are commonly used in optical communications systems conforming to the SDH (Synchronous Digital Hierarchy) and SONET (Synchronous Optical Network) standards. One requirement of the physical layer standards is that the transmitter must exhibit a Side Mode Suppression Ratio (SMSR) of at least 30 dB, which means that the power of the single dominant transmission mode must be at least 30 dB greater than that of each of the other transmitted modes.
An observed problem with some SLM lasers is that at low temperatures the gain peak, which is defined by the semiconductor band gap, shifts to a lower wavelength than that of the device grating. This can lead to some modes near to the gain peak having a power that breaches the minimum SMSR requirement. It is an object of the present invention to provide a semiconductor laser arrangement that addresses this problem.
A similar problem that may be seen in SLM lasers occurs at high temperatures, where the gain peak is at a longer wavelength than the grating wavelength, leading to a breach of the SMSR requirement in a similar way. If the suppression of the modes near the gain peak is not sufficient, one or more modes near the gain peak may have a power that breaches the minimum SMSR requirement. The present invention additionally provides a means of addressing this problem alone or in conjunction with the low-temperature problem outlined above.